21,390 research outputs found
Turbofan aft duct suppressor study program listing and user's guide
A description of the structure of the Annular Flow Duct Program (AFDP) for the calculation of acoustic suppression due to treatment in a finite length annular duct carrying sheared flow is presented. Although most appropriate for engine exhaust ducts, this program can be used to study sound propagation in any duct that maintains annular geometry over a considerable length of the duct. The program is based on the modal analysis of sound propagation in ducts with axial segments of different wall impedances. For specified duct geometry, wall impedance, flow and acoustic conditions in the duct (including mode amplitude distribution of the source) and duct termination reflection characteristics, the program calculates the suppression due to the treatment in the duct. The presence of forward and backward traveling modes in the duct due to the reflection and redistribution of modes at segment interfaces and duct end terminations are taken into account in the calculations. The effects of thin wall boundary layers (with a linear or mean flow velocity profile) on the acoustic propagation are also included in the program. A functional description of the major subroutines is included and a sample run is provided with an explanation of the output
Robust model-based controller synthesis for the SCOLE configuration
The design of a robust compensator is considered for the SCOLE configuration using a frequency-response shaping technique based on the LQG/LTR algorithm. Results indicate that a tenth-order compensator can be used to meet stability-performance-robustness conditions for a 26th-order SCOLE model without destabilizing spillover effects. Since the SCOLE configuration is representative of many proposed spaceflight experiments, the results and design techniques employed potentially should be applicable to a wide range of large space structure control problems
Application of the LQG/LTR technique to robust controller synthesis for a large flexible space antenna
The problem of synthesizing a robust controller is considered for a large, flexible space-based antenna by using the linear-quadratic-Gaussian (LQG)/loop transfer recovery (LTR) method. The study is based on a finite-element model of the 122-m hoop/column antenna, which consists of three rigid-body rotational modes and the first 10 elastic modes. A robust compensator design for achieving the required performance bandwidth in the presence of modeling uncertainties is obtained using the LQG/LTR method for loop-shaping in the frequency domain. Different sensor actuator locations are analyzed in terms of the pole/zero locations of the multivariable systems and possible best locations are indicated. The computations are performed by using the LQG design package ORACLS augmented with frequency domain singular value analysis software
Exotic attractors of the non-equilibrium Rabi-Hubbard model
We explore the phase diagram of the dissipative Rabi-Hubbard model, as could
be realized by a Raman-pumping scheme applied to a coupled cavity array. There
exist various exotic attractors, including ferroelectric, antiferroelectric,
and inccomensurate fixed points, as well as regions of persistent oscillations.
Many of these features can be understood analytically by truncating to the two
lowest lying states of the Rabi model on each site. We also show that these
features survive beyond mean-field, using Matrix Product Operator simulations.Comment: 5pages, 3 figures, plus supplementary material. Final version, as
publishe
Prediction of long and short time rheological behavior in soft glassy materials
We present an effective time approach to predict long and short time
rheological behavior of soft glassy materials from experiments carried out over
practical time scales. Effective time approach takes advantage of relaxation
time dependence on aging time that allows time-aging time superposition even
when aging occurs over the experimental timescales. Interestingly experiments
on variety of soft materials demonstrate that the effective time approach
successfully predicts superposition for diverse aging regimes ranging from
sub-aging to hyper-aging behaviors. This approach can also be used to predict
behavior of any response function in molecular as well as spin glasses.Comment: 13 pages, 4 figure
Gamma-ray bursts as the birth-cries of black holes
The origin of cosmic gamma-ray bursts remains one of the most intriguing
puzzles in astronomy. We suggest that purely general relativistic effects in
the collapse of massive stars could account for these bursts. The late
formation of closed trapped surfaces can occur naturally, allowing the escape
of huge energy from curvature-generated fireballs, before these are hidden
within a black hole.Comment: 4 pages Revtex, 1 figure. This essay received an honorable mention in
the Gravity Research Foundation essay competitio
Mappings preserving locations of movable poles: a new extension of the truncation method to ordinary differential equations
The truncation method is a collective name for techniques that arise from
truncating a Laurent series expansion (with leading term) of generic solutions
of nonlinear partial differential equations (PDEs). Despite its utility in
finding Backlund transformations and other remarkable properties of integrable
PDEs, it has not been generally extended to ordinary differential equations
(ODEs). Here we give a new general method that provides such an extension and
show how to apply it to the classical nonlinear ODEs called the Painleve
equations. Our main new idea is to consider mappings that preserve the
locations of a natural subset of the movable poles admitted by the equation. In
this way we are able to recover all known fundamental Backlund transformations
for the equations considered. We are also able to derive Backlund
transformations onto other ODEs in the Painleve classification.Comment: To appear in Nonlinearity (22 pages
Role of initial data in spherical collapse
We bring out here the role of initial data in causing the black hole and
naked singularity phases as the final end state of a continual gravitational
collapse. The collapse of a type I general matter field is considered, which
includes most of the known physical forms of matter. It is shown that given the
distribution of the density and pressure profiles at the initial surface from
which the collapse evolves, there is a freedom in choosing rest of the free
functions, such as the velocities of the collapsing shells, so that the end
state could be either a black hole or a naked singularity depending on this
choice. It is thus seen that it is the initial data that determines the end
state of spherical collapse in terms of these outcomes, and we get a good
picture of how these phases come about.Comment: 5 pages, Revtex4, Revised version, To appear in Physical Review
Turbofan aft duct suppressor study. Contractor's data report of mode probe signal data
Acoustic modal distributions were measured in a fan test model having an annular exhaust duct for comparison with theoretically predicted acoustic suppression values. This report contains the amplitude and phase data of the acoustic signals sensed by the transducers of the two mode probes employed in the measurement. Each mode probe consisted of an array of 12 transducers sensing the acoustic field at three axial positions and four radial positions
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